The answer, as given by generations of airship-loving engineers, has been that it just isn't feasible. Even the strongest tanks can only hold so much pressure; given the immense volumes of helium which would need to be crammed away in order to cope with a large cargo being dropped off, the storage tanks required would still be very large - and thus tremendously heavy, as they'd need to be made of very strong stuff. Then, in order to compress the helium in a reasonable amount of time, massive compressors would be necessary - with hefty transmissions to carry power from the ship's engines.

But Igor Pasternak reckons he's cracked this with his mysterious COSH technology - which he says was developed for DARPA and tested successfully this summer aboard a normal non-rigid blimp. According to Pasternak, "this system works by compressing, storing then decompressing helium within the envelope to adjust the vehicle’s buoyancy" (our emphasis).

Looking at Aeros' pic of a COSH-equipped blimp as compared to one without, the difference seems to be a pair of large doughnut-like affairs around the outside. One might speculate that Pasternak is using modern materials to make these contain helium at relatively low pressures - just enough that it becomes significantly more dense, so forfeiting buoyancy to the tune of several times the tanks' own volume and perhaps adding weight to boot if the tanks are strong enough. A toroid shape like the ones pictured is particularly efficient at containing pressure, and could add stiffness to the ship as well. As for the mysterious cylindrical machinery shown by Aeros to illustrate COSH, who knows? But it might perhaps be an efficient compressor/condenser for pushing gas into the toroid tanks. These would, in a rigid ship, be placed within the envelope.

Same blimp, with COSH. External lo-pressure tanks?

However it works, if Pasternak can be believed, he has achieved what such legendary engineers as Hugo Eckener and Frank Piasecki regarded as impossible; and one of the greatest weaknesses of the airship has been solved. A titanic "Aeroscraft" should indeed be able to haul an entire US Army unit with all its supplies - more than 500 tons of load - around the world in days and drop it off in the middle of a desert, without losing any of its valuable helium.

Well, maybe. Specifics on the COSH kit aren't forthcoming, but it seems clear that in fact even Pasternak doesn't think it could compensate on its own for the sudden disembarkation of several hundred tons, as in the "Walrus" military mega-ship once planned by DARPA.

The Walrus, on a long-range mission, would probably have taken off in a very heavy condition - Pasternak describes his Aeroscraft as "a heavier than air design". On departure, the ship would use its engine exhaust to superheat its helium cells, making them expand to fill the whole hull and gaining extra lift. The takeoff would probably be a rolling one, generating still more lift from the craft's wings and body to get airborne.

On arrival, the ship could set down vertically supported only by its props, swiveled to point upward; it would have lost a lot of weight in burned fuel, and wouldn't need forward-motion dynamic lift to stay aloft. Then, chilling systems would cool the helium, shrinking the cells and cutting buoyancy drastically. Air drawn into the ship as a result would also be chilled down, making it heavier and reducing lift still further. The COSH miracle-compressor equipment would also be doing its part.

All this, according to Pasternak, would mean that after a reasonable amount of time the soldiery and all their heavy equipment could safely drive off the ramps without fear of the mighty airship suddenly lurching uncontrollably upwards as they did so. But there would still be no need to vent off expensive helium.

Perhaps a new kind of compressor?

But the return of great airships remains uncertain. The DARPA Walrus was cancelled in 2006, though the agency did hand out some cash to Pasternak to demonstrate his COSH gear. Apart from that, however, Aeros has had to content itself with its established business making and leasing out ordinary nonrigid balloon-type blimps, aerostats etc. - mostly for advertising purposes.

The recent award of more DARPA cash to develop Aeros' super-strong yet super-light rigid structure tech could be taken to mean that some heavy-lift airship diehards are still clinging on at DARPA - but probably not. As Wired magazine points out, the $2m seems to have been budgeted as the result of a Congressional "earmark," by a Californian politician representing Aeros' home territory. The money would seem to have appeared as a matter of porkbarrel politics rather than genuine military enthusiasm.

That's not to say that Pasternak can't do what he says; the Walrus may not have been cancelled because it couldn't, in DARPA's view, be built. Even if it worked exactly as described on the tin, Walrus would have been a bit marginal from a military point of view. With a maximum pressure ceiling of 10,000 feet - probably less in reality - and top speed of only 140 knots, the enormous machine would be an easy target even to lightweight shoulder-fired missiles along its entire flight path.

The Aeroscraft is now being offered in a smaller - though still pretty huge - design, the rather yummy looking ML866 (Flash!). This design has been going through initial FAA acceptance since March. The ML866 can be set up as a nifty flying office or conference suite; a sort of aerial bateau-mouche restaurant/observation cruiser; or - of course - a private luxury sky yacht. Alternatively Aeros will do you a bigger commercial cargo job, though not of the size that DARPA wanted - "up to 60 tons" of payload are on offer.

Let's hope that some rather less boring billionaire, tired of dull water megayachts, space tourism etc., comes forward and gives Pasternak a chance to really show what he can do. ®